Heretofore, known instruments for aiding in the use of maps, such as exemplified in the Hotchkiss U.S. Pat. No. 4,337,577 dated July 6, 1982 and Lambert U.S. Pat. No. 4,359,628 dated Nov. 16, 1982, are very limited in their usefulness, particularly for military purposes. One such limitation is that they are applicable to a single scale map because each instrument is calibrated for use with one of several military scaled maps, as for example 1:25,000, 1:50,000 or 1:125,000. Another limitation of known instruments, particularly for military uses, is that the user has a need in the effective use of a map to make arithmatical computations and, therefore, military recruits must undergo extensive training in map reading. Some of the arithmatical computations involve converting magnetic azimuths to grid azimuths and conversion to back azimuths when it is desired to locate on the map an unknown point from two known points, which functions are referred to as "resection" and "intersection." Also, in determining the total distance along a sinuous path, e.g. a road or railroad track, the user must add the straight portions of the path between the curved portions. A further limitation of known map reading devices is that the artillery uses instruments in which angular directions are measured in mils rather than degrees for more accurate aiming of the weapons. Thus, artillery personnel and those spotting for artillery units must have special map reading and plotting instruments. Obviously, where in the use of heretofore known map reading instruments computations are required, the need for paper and/or a writing implement poses a problem, particularly under actual field conditions where paper and a writing tool is not ,always available to the map user. Also, map reading and plotting instruments of heretofore known types require the user to draw lines on a map and, in absence of available paper, the user may use the map for making computations. These writings on a map lead to short map life and leaves marks which, even if erased, are visible or can be made visible and may give aid to an enemy if the map is captured. These aforementioned limitations and others have been eliminated by the map reading and plotting instrument of the present invention wherein the instrument has a more universal use, automatically performs the calculations and can be used by a person having minimal training in its use. Also, it is desirable, particularly for military use, that the map reading and plotting instrument be of relatively rugged construction and compactness. It should also be made of a transparent material to permit map features to be seen when the instrument, in its use, is placed on the map.
Accordingly, it is an object of this invention to provide an improved map reading and plotting instrument which is capable of being used with a plurality of maps of different scale. Another object of the present invention is to provide an improved map reading and plotting instrument which is compact and sturdy and is therefore particularly useful in the military. A further object of this invention is to provide an improved map reading and plotting instrument which can be effectively used by persons having very little instructions in its use. A still further object of this invention is to provide an improved map reading and plotting instrument capable of performing a multiplicity of functions automatically.